Powassan Encephalitis in Maine

By Amesh A. Adalja, FACP, FACEP, January 10, 2014

As diagnostic testing for unexplained central nervous system infections such as meningitis and encephalitis increases, the true spread of the viruses that cause these infections will become more apparent. Such a scenario is likely responsible for the identification of Powassan encephalitis in Maine last month.

Tick Borne Encephalitis

The Powassan virus, and the disease it causes, is classified as a part of the Tick Borne Encephalitis (TBE) virus species. TBE includes 3 subtypes: Far Eastern, Siberian, and European, along with Powassan, Kyasanur Forest Virus, Omsk hemorrhagic fever, and louping ill virus.1

Powassan virus is further divided into 2 lineages, each transmitted by different ticks: lineage 1 by the woodchuck (I. cookei) or the squirrel tick (I.marxi), lineage 2 by the deer tick (I.scapularis). Lineage 2 is sometimes called deer tick virus. The predominant route of human transmission is via deer ticks.2

10% of Cases Are Fatal

Since many individuals infected with Powassan virus are asymptomatic, the 50 cases of Powassan encephalitis reported each year are an underestimate of the true viral burden. The cases that are reported are clustered in the northeastern states and the Great Lakes region. Symptomatic patients present 1 week to 1 month after infection with general symptoms of encephalitis, which include fever, headache, altered mental status, and seizures. Of those who are symptomatic, 50% present with focal neurologic deficits and some progress to spinal paralysis. Fatalities occur in 10% of cases.1,2

In the US, there is no treatment or FDA-approved vaccine, though vaccines are available in Europe.1

Postmortem Diagnosis

Like other causes of viral encephalitis, there is no rapid or 100% sensitive test to diagnose Powassan encephalitis. Serology may be useful. Viral isolation or PCR-positive results usually come from postmortem samples.2

What this case illustrates is the difficulty in arriving at a specific microbiologic diagnosis in a patient presenting with encephalitis. The majority of presumed infectious encephalitis cases do not have an identified etiology,3 reinforcing the fact that our diagnostic ability in this arena is not optimal. As only 1 type of viral encephalitis has a specific treatment—acyclovir for herpes encephalitis—pathogen-specific diagnosis may seem superfluous. However, given the link of certain encephalitis viruses to specific vector tick species, which can be combated with interventions and heightened public awareness, diagnosis is never superfluous.